The primary mode of action of phytogenic feed additives arises from beneficially affecting the ecosystem of gastrointestinal microbiota through controlling potential pathogens. Improved digestive capacity in the small intestine may be considered an indirect side effect of phytogenic stabilizing the microbial eubiosis in the gut. Consequently, phytogenic relieve the host animals from immune defense stress during critical situations and increase the intestinal availability of essential nutrients for absorption, thereby helping animals to grow better within the framework of their genetic potential.

ANTIBACTERIAL ACTIVITY OF PHYTOBIOTICS

Herbs and spices are well known to exert antimicrobial actions in vitro against important pathogens including fungi (Windisch et al., 2008). A common feature of phytobiotics is that they are a very complex mixture of bioactive components. For example, hawthorn fruit, a common growth-enhancing and digestion modifier has been shown to contain >70 kinds of organic chemicals along with some unidentified factors and active bio-active compounds (Wang et al., 1998).

Growth enhancement through the use of phytobiotics is probably the result of the synergistic effects among complex active molecules existing in phytobiotics. Phytochemicals in phytobiotics are well known to have antimicrobial ability (Cowan, 1999). Phytochemicals exert their antimicrobial activity through different mechanisms, tannins for example act by iron deprivation, hydrogen bounding or non specific interactions with vital proteins such as enzymes (Scalbert, 1991).

Chung et al. (1993) showed that tannic acid inhibits the growth of intestinal bacteria such as Bacteroides fragilis, Clostridium perfringens, E. coli and Enterobacter cloacae. Alkaloid is known to be a DNA intercalator and an inhibitor of DNA synthesis through topoisomerase inhibition (Karou et al., 2006). The main mechanism by which saponins display an antimicrobial activity is based on their ability to form complexes with sterols present in the membrane of microorganisms.This causes damages in the membrane and the consequent collapse of cells (Morrissey and Osbourn, 1999). Essential oils have long been recognized for their anti-microbial activity (Lee et al., 2004a) and they have gained much attention for their potential as alternatives to antibiotics in broiler chickens. Some studies with broilers demonstratedin vivo antimicrobial efficacy of essential oils against Escherichia coli and Clostridium perfringens (Jamroz et al., 2003; Mitsch et al., 2004).

The exact anti-microbial mechanism of essential oils is poorly understood. However, it has been suggested that their lipophilic property (Conner, 1993) and chemical structure (Farag et al., 1989a, b) can play a role. It was suggested that terpenoids and phenylpropanoids can penetrate the membranes of the bacteria and reach the inner part of the cell because of their lipophilicity (Helander et al., 1998). Moreover, structural properties, such as the presence of the functional groups (Farag et al., 1989c) and aromaticity (Bowles and Miller, 1993) are also responsible for the antibacterial activity of essential oils.

PHYTOGENICS AS FEED ADDITIVES IN POULTRY

In recent years, phytogenic feed additives have attracted increasing interest as an alternative feeding strategy to replace antibiotic growth promoters. Hernandez et al. (2004) reported that extracts from sage, thyme and rosemary and the blend of carvacrol, cinnamaldehyde and capsaicin improved feed digestibility in broilers. The researchers attributed the positive effects of plant extracts on nutrient digestibility to the appetite and digestion-stimulating properties and antimicrobial effects. Therefore they may exert multiple functions in the animal body. Increased feed intake and digestive secretions are also observed in animals offered phytobiotic-supplemented feed (Windisch and Kroismayr, 2006).